Tree Size Diversity Research Articles

Development and dominance of Douglas-fir in North American rainforests

One of the five tallest tree species, Pseudotsuga menziesii has enormous economic and ecological importance, but rainforests dominated by this species are not as well understood as their drier montane counterparts. We climbed and measured 30 trees up to 97 m tall growing in coastal forests of the Olympic Peninsula and northern California to quantify structural attributes—leaves, bark, cambium, sapwood, heartwood, deadwood, biomass, growth increments, and age—and combined these with an equal number of trees up to 85 m tall growing in forests of the Cascade Mountains to develop allometric equations based on ground-level predictors. After comparing new equations to those previously published, we applied the best available equations for tall forests to predict aboveground quantities of all vascular plant species in 12 ha of Olympic and Cascade forests. The largest (117 Mg) and one of the oldest (615 years) trees we studied had the highest biomass increment (305 kg yr−1), but age had a negative effect on current and long-term growth increments. After accounting for variation in tree size and aboveground vigor, older trees produced less wood annually and grew less efficiently than younger trees. Size of P. menziesii trees increased more rapidly, and the proportion of biomass and leaf area in P. menziesii decreased more slowly, in Olympic than Cascade forests over six centuries following stand-replacing fire. Maximum aboveground biomass (1999 Mg ha−1) and carbon density (994 Mg ha−1) occurred in a Cascade forest with high abundance of three conifer species (P. menziesii, Tsuga heterophylla, Thuja plicata), but maximum P. menziesii biomass (1289 Mg ha−1) occurred in an Olympic forest with 50 trees ha−1 up to 90 m tall. Vulnerability to wood decay fungi and dependence on fire for stand dominance limit P. menziesii biomass accumulation in rainforests.

Site and tree factors determining the distribution of Phellinus tremulae in Populus tremuloides in Utah, USA

Populus tremuloides Michx. is an iconic tree of the mountains of the western United States. In Utah, it very commonly suffers from white trunk rot caused by Phellinus tremulae (Bondartsev) Bondartsev & P.N.Borisov. The incidence of this disease is affected by tree and site characteristics, but the magnitude of these effects appears to be site-dependent. To minimize harvest wood loss in Utah, we determined the locally important factors that explain the wide variation in the incidence of sporocarps of Phellinus tremulae. To avoid confounding of factors, we utilized a multiple regression approach. We found that while the incidence of Phellinus tremulae sporocarps on quaking aspen was always low at high elevations, it was variable at low elevations. Our logistic regression model indicated that variation in the incidence of sporocarps at low elevations was attributable, in part, to variation in aspect, slope, environmental stress, and tree age and size. Based on these results, we recommend that harvesting at elevations below 2500 m be confined to younger trees or to sites on relatively steep, north-facing slopes. Because of site-dependency, the same general method could be used to establish harvesting criteria in other regions.

The effects of burning season and severity on the mortality over time of Pinus nigra spp. salzmannii (Dunal) Franco and P. sylvestris L.

Abstract Understanding tree mortality after fires can be used to improve planning of prescribed burning and post-burning salvage logging. This study aims to assess the effects of burning season, fire severity and tree size on the evolution of post-burning mortality of pine species with different fire resistance traits. In 2013, two paired plots (1 ha each) were burned, one in spring and the other in fall, in two mixed-stands of P. sylvestris and P. nigra, and the fire severity and tree size variables were measured. Mortality was assessed individually in a total of 933 pines during the following 3 years post-burning. We modeled the initial (1 year post-burning) and delayed (3 years post-burning) tree mortality using Firth logistic regression and Conditional Inference trees. The probability of dying after burning was lower in P. nigra than in P. sylvestris confirming that P. nigra has higher fire-resistance compared to the moderate fire-resistance of P. sylvestris . Initial mortality was slightly higher for pines burned in spring than for those burned in fall; however, after 3 years the proportion of pines that had died after the fall burns was higher than that after the spring burns. Nevertheless, for the two periods modeled, the likelihood of a pine dying was significantly higher in spring than in fall given a certain level of crown scorch. This suggests that pines with damaged crowns were more likely to die when burning occurred during periods of active growth. However, the higher fire severities at the base of the trees that occurred during the fall burns compared to the spring burns are probably the cause of the higher rates of delayed tree mortality. Higher likelihood of mortality in trees burned in fall, with higher percentages of white ashes in the stem base suggest basal stem girdling as a cause of delayed pine mortality. We can conclude that crown damage is more detrimental when burning occurs during the growth season, while longer combustion times during fall burns may have caused the higher delayed pine mortality in comparison with spring burns.

Variation in oil content and tree size between six geographically separate Santalum spicatum families, established near Narrogin, Western Australia

ABSTRACTSix geographically separate sandalwood (Santalum spicatum) tree families were established together in a trial near Narrogin, Western Australia to compare oil content and tree size variation at ages 10 and 18 years. The trial site contained a total of 300 sandalwood trees with 41–55 trees from each family. All 300 sandalwood trees were measured for tree size: height, stem diameter and bole length. Wood core samples (heartwood + sapwood) were also taken at 300 mm above the ground from 71 to 72 separate trees, at ages 10 and 18 years. Each wood core sample was ground separately and then a sub-sample was analysed for oil concentration, and α- and β-santalol concentration within the oil.The mean extractable oil concentrations within the sandalwood stems at 300 mm above the ground were significantly greater from the Borden and Dumbleyung families (1.1–1.6%) than the Ravensthorpe family (0.5–0.8%), at both ages 10 and 18 years. However, the oil quality (mean concentration of α- and β-santalol) was not significantly different between the six families at ages 10 or 18 years. Instead, oil quality was highly variable within each family.Between sandalwood ages 10 and 18 years, the overall mean extractable oil concentration from the six different families increased significantly from 0.8 to 1.3%. During the same time period, mean α-santalol concentration increased significantly from 8.6 to 13.8%, and mean β-santalol concentration also increased significantly from 3.3 to 4.5%.Mean tree height, stem diameter and bole length varied significantly between the six families at age 18 years. The Borden family appeared to be more tree-like in form, with a significantly longer bole length, than some of the other families which were more shrub-like in appearance.

Group Opening Outcomes, Sustainable Forest Management, and the Menominee Nation Lands

Ideally, variants of single-tree, group, and patch selection create new, spatially aggregated age classes and maintain a diversity of tree species and sizes in multiaged, mixed-species forests. We explored this notion in northern hardwood forests on the Menominee Nation, a forest ecosystem without the exploitive cutting history of most forests in the western Great Lakes region. Although the outcomes suggested a lack of relationship between gap characteristics and tree density, the expectations for tree regeneration were largely met: gap tree densities were >600 stems/ac and predominantly composed of sugar maple (Acer saccharum), American elm (Ulmus americana), and yellow birch (Betula alleghaniensis). Examination of stand diameter distributions indicate that gaps may not be necessary to establish regeneration on Menominee forests. To deepen the interpretation of our results, we include field and office discussions regarding the practicality of group openings when managing this forest.

Structure and Spatial Distribution of Dead Wood in Two Temperate Old-Growth Mixed European Beech Forests

Old-growth forests are often looked at as reference for close to nature silviculture, which aims to manage forests in a natural way. An important component of these forests is the large amount of deadwood they possess; the role of dead wood in the forest ecosystem has been well recognised. A detailed investigation of dead wood characteristics (the amount per stand, species, dead wood type, decay class, size and spatial distribution) was performed in two Romanian old-growth European mixed beech forests: Runcu Grosi Reserve (sessile oak-beech) and Sinca (silver fir-beech). Dead wood pieces were classified as belonging to one of seven dead wood types and one of five decay classes. The total amount of the dead wood was greater in Runcu Grosi (240.8 m3 ha-1) than in Sinca Forest (135.5 m3 ha-1). The majority of the dead wood in Runcu Grosi was composed of sessile oak (91.7%), whereas in Sinca Forest, the main dead wood species was silver fir (67.0%); both species exhibited higher values of probability density than beech, the second most important species. The dead wood exhibited much variation in tree size and in dead wood type, and covered the entire spectrum of decomposition classes. The main spatial pattern of all standing dead trees was random in both forests (over 55%), with a reduced participation of regularity and aggregation. The results of this investigation could be used as key values derived from natural conditions to enhance the biodiversity related to dead wood in managed temperate beech mixed forests.

Effect of forest stand management on species composition, structural diversity, and productivity in the temperate zone of Europe

Forests in Central Europe have been used since centuries and were systematically changed by silvicultural practices since the late 1700s. Focusing on possible biodiversity losses, forest stand management primarily impacts on the biodiversity attributes ‘composition, structure and functioning’ that are interlinked and incorporate different organizational and spatial ecosystem levels. Forest stand management can manipulate tree species diversity, which in turn affects stand structure and finally leads to changes in forest functions and services. We used meta-analyses to review 49 papers with 197 comparisons on compositional and structural diversity between unmanaged and managed forests as well as 53 comparisons of biomass productivity between mixed and monoculture forests throughout the temperate biome of Europe. Besides management impacts, for species and structural comparisons, we took time since abandonment of forest stand management into consideration when dealing with unmanaged forests. For species richness and diversity, we could not detect significant management effects; no significant influences of time since abandonment of management could be identified either. Structural attributes partly revealed considerable responses to forest stand management. Importantly, deadwood quantity, the number of microhabitats, and tree size diversity were reduced by forest stand management, while no effect was found for stand basal area and the number of living trees. Some of these effects become more pronounced with increasing time since abandonment. Forest productivity was significantly enhanced by regular moderate thinning compared with untreated stands and by tree species mixing compared with monospecific stands. Based on these results, we discuss the role of direct and indirect silvicultural effects, interactions between biodiversity components, the problem of defining the unmanaged reference state, and perspectives for forest stand management in practice.

The forest strata-dependent relationship between biodiversity and aboveground biomass within a subtropical forest

The relationships between biodiversity and aboveground biomass in forest ecosystems have been intensively studied in recent decades. Still, the mechanisms that underlie it remain highly debated. We hypothesized that overstorey species diversity and individual tree size variation contribute to aboveground biomass and understorey species diversity through the niche complementarity effect, while weaken the relationship between understorey aboveground biomass and individual tree size variation due to the mixed effects of tree development, biotic interaction and reduced available resources by overstorey strata. The integrative relationships of species diversity and tree size variation (variation in diameter at breast height-DBH) with aboveground biomass were analysed at overstorey and understorey strata across 125 plots in a 5-ha subtropical forest in Eastern China. For comparison, we tested these relationships at individual strata (isolation modelling), and whole-community level, by using linear structural equation model while accounting for the effects of soil nutrients. The integrative modelling accounted for 35, 31, 16, 12, 4, and 0% of the variation in understorey aboveground biomass, overstorey aboveground biomass, overstorey DBH variation, overstorey species diversity, understorey species diversity, and understorey DBH variation, respectively. Overstorey DBH variation and species diversity had the positive direct effects on overstorey aboveground biomass. Overstorey species diversity significantly promoted the understorey species diversity, but DBH variation and aboveground biomass of overstorey strata had negligible effects on the diversity and aboveground biomass of understorey strata. Soil nutrients had positive direct effect on overstorey DBH variation, but negative direct effects on overstorey and understorey aboveground biomass and overstorey species diversity. These results provide strong evidence for the niche complementarity effect for driving positive relationships of species diversity and individual tree size variation with aboveground biomass at overstorey strata. The strong and consistent negative effects of soil nutrients on overstorey aboveground biomass and species diversity suggest an important mechanism that high species diversity of overstorey strata with great tree size variation on nutrient-poor soils is crucial for driving high aboveground biomass in subtropical forest ecosystems. In conclusion, this study suggests that no sole and ubiquitous relationship between biodiversity and aboveground biomass exists in a structurally complex forest, but rather that the magnitude and direction of this relationship is greatly dependent on the forest strata where available resources shift substantially. We argue that ecological models for predicting aboveground biomass would be improved by including separate effects of overstorey and understorey diversity.

Avian diversity in Norway spruce production forests – How variation in structure and composition reveals pathways for improving habitat quality

Forests used for timber production provide essential ecosystem services to society, as well as potential breeding habitat for bird communities. In southern Sweden, 90% of productive forest land is used for timber production and stands dominated by Norway spruce (Picea abies) constitute approximately 40% of the forested area. Due to their homogeneous structure, these spruce production forests are often regarded as depauperate. Despite this perception, knowledge about the biodiversity found in these stands is scarce. Here we synthesize the results of four separate bird surveys conducted within 35 spruce production stands of southern Sweden. The results are compared to recent population trends within the general study area. In total 49 bird species were recorded, with a strong difference in species composition between newly planted clear-cuts (forest age <15years) and forests older than 15years. The majority of species encountered in the older forest category were common forest birds, with a single red-listed species among the regularly occurring species. In contrast, three red-listed “farmland species” were frequently encountered in the newly planted forests, revealing the capacity of those birds preferring open and recently disturbed habitats to utilize clear-cuts. A higher diversity of tree sizes and the inclusion of even relatively small proportions (<15%) of broadleaved tree species had a positive effect on bird species richness. Several species encountered in spruce production forests are declining in numbers, but it is not clear whether these stands are acting as source or sink environments for their populations. However, our results indicate that relatively small adjustments to spruce forest management should improve the quality of this widespread habitat.

InvasiveSyzygium jambostrees in Puerto Rico: no refuge from guava rust

Abstract:Biological invasions can have negative consequences for resident biota, particularly when disease-causing organisms are involved.Austropuccinia psidii, or guava rust, has rapidly spread through the tropics affecting both native and non-native Myrtaceae. In Puerto Rico, the rust has become common onSyzygium jambos, an invasive tree native to South-East Asia. What are the drivers of infection, and do refugia exist across a heterogeneous landscape? We address these questions using species distribution modelling and beta regressions. The realized and potential distribution ofSyzygium jambosis extensive. The model produced an AUC of 0.88, with land-use categories and precipitation accounting for 61.1% of the variation. Predictability ofS. jambosis highest in disturbed habitats, especially in mountainous regions with high precipitation. All 101 trees surveyed and measured across Puerto Rico showed signs of infection to varying extents. Infection severity was consistently associated with annual mean temperature in all top beta regression models, but was also commonly associated with tree size and precipitation variables. We found no safe sites forS. jambos. Many trees were extremely unhealthy and some were dead, suggesting thatS. jambosmay soon become extinct on the island or reduced to persistent stump sprouts. Native vegetation may benefit from the local demise ofS. jambos. While the rust has not jumped to native Myrtaceae, vigilance is required, as host-shifts have occurred in other tropical regions.

Disentangling the effects of species diversity, and intraspecific and interspecific tree size variation on aboveground biomass in dry zone homegarden agroforestry systems

The biodiversity – aboveground biomass relationship has been intensively studied in recent decades. However, no consensus has been arrived to consider the interplay of species diversity, and intraspecific and interspecific tree size variation in driving aboveground biomass, after accounting for the effects of plot size heterogeneity, soil fertility and stand quality in natural forest including agroforests. We tested the full, partial and no mediations effects of species diversity, and intraspecific and interspecific tree size variation on aboveground biomass by employing structural equation models (SEMs) using data from 45 homegarden agroforestry systems in Sri Lanka. The full mediation effect of either species diversity or intraspecific and interspecific tree size variation was rejected, while the partial and no mediation effects were accepted. In the no mediation SEM, homegarden size had the strongest negative direct effect (β=−0.49) on aboveground biomass (R2=0.65), followed by strong positive direct effect of intraspecific tree size variation (β=0.32), species diversity (β=0.29) and interspecific tree size variation (β=0.28). Soil fertility had a negative direct effect on interspecific tree size variation (β=−0.31). Stand quality had a significant positive total effect on aboveground biomass (β=0.28), but homegarden size had a significant negative total effect (β=−0.62), while soil fertility had a non-significant total effect on aboveground biomass. Similar to the no mediation SEM, the partial mediation SEMs had explained almost similar variation in aboveground biomass because species diversity, and intraspecific and interspecific tree size variation had non-significant indirect effects on aboveground biomass via each other. Our results strongly suggest that a multilayered tree canopy structure, due to high intraspecific and interspecific tree size variation, increases light capture and efficient utilization of resources among component species, and hence, support the niche complementarity mechanism via plant-plant interactions.

Identification of determining traits of seed production in Carapa procera and Pentadesma butyracea, two native oil trees from riparian forests in Burkina Faso, West Africa

Natural resources are of vital importance for populations of African rural communities. Native oil trees are sources of promotable products for which the natural resource availability is urgently needed. The present study aims to assess the seed production of two oil tree species, Carapa procera and Pentadesma butyracea to identify traits related to seed production. Seeds were collected from 77 trees of C. procera and 36 trees of P. butyracea and, stem diameter, height and crown diameters were measured. Ten mature fruits per tree were collected randomly and seeds were extracted, counted, dried and weighed to obtain dry weight. Analyses of variance were performed to test for differences in seed production between years and between size classes of trees. To predict seed production, best subsets regression was used to perform predictive allometric equations by selecting tree size variables for appropriate models. The seed production per tree of C. procera and P. butyracea was 1–11 kg depending on tree size. For both species, there was a significant variation (p < 0.05) among stem diameter classes, with the highest seed weight found in the >50 cm classes. Seed production did not vary significantly between years. The regression equation models performed for the prediction of seed production in both species were statistically significant (p < 0.05). They showed high goodness-of-fit (R2 > 70%) and revealed good predictive accuracy for new observations. The predictive equations established are useful tools for the planning of rational seed exploitation for both native oil trees.

Structural diversity of selected oak stands (Quercus robur L.) on the Krotoszyn Plateau in Poland

Abstract The oak forests growing on the Krotoszyn Plateau are the biggest and most valuable oak forest complexes in Poland. The purpose of this study was to analyze the structural diversity of oak (Q. robur L.) stands older than 140 years. Two natural stands located within the forest reserve as well as two managed stands were chosen for investigation. Species and tree size diversity were analyzed using different measures and spatially explicit and inexplicit indices. The results indicate that the structural diversity of the protected oak forests did not differ significantly from that of managed stands. On the contrary, undisturbed natural processes in protected forests can result in a low diversity of some structural attributes in such stands. Although most indices pointed towards a higher structural diversity in protected forests, the differences were not large, especially in the case of spatially explicit indices. The spatial arrangement of live trees was either random, aggregated or regular, with the latter clearly dominating managed, poorly mixed oak stands. Furthermore, a lack of natural disturbances strong hornbeam expansion, in which case strict protection is not favorable for oak sustainability as it creates unsuitable conditions for oak regeneration beneath the hornbeam canopy.

Intraspecific variability in growth response to environmental fluctuations modulates the stabilizing effect of species diversity on forest growth

SummaryDifferences between species in their response to environmental fluctuations cause asynchronized growth series, suggesting that species diversity may help communities buffer the effects of environmental fluctuations. However, within‐species variability of responses may impact the stabilizing effect of growth asynchrony.We used tree ring data to investigate the diversity–stability relationship and its underlying mechanisms within the temperate and boreal mixed woods of Eastern Canada. We worked at the individual tree level to take into account the intraspecific variability of responses to environmental fluctuations.We found that species diversity stabilized growth in forest ecosystems. The asynchrony of species’ response to climatic fluctuations and to insect outbreaks explained this effect. We also found that the intraspecific variability of responses to environmental fluctuations was high, making the stabilizing effect of diversity highly variable.Synthesis. Our results are consistent with previous studies suggesting that the asynchrony of species’ response to environmental fluctuations drives the stabilizing effect of diversity. The intraspecific variability of these responses modulates the stabilizing effect of species diversity. Interactions between individuals, variation in tree size and spatial heterogeneity of environmental conditions could play a critical role in the stabilizing effect of diversity.

Dynamics of ecosystem services in Pinus sylvestris stands under different managements and site quality classes

Timber and biodiversity are considered two antagonistic ecosystem services (ES), largely influenced by silviculture and site ecological conditions. In order to address the trade-offs between these two ES over time, we performed a retrospective study at compartment level in the Pinus sylvestris forests of the Spanish Central Mountain System. Archival data from Management Plans for eight forests with contrasting silvicultural systems (uniform shelterwood system, group shelterwood system, irregular shelterwood system,) and three different site quality classes were analysed. Timber production was assessed through stand volume, harvested timber volume and a stand volume index. Biodiversity was examined through structural diversity (Gini index, Shannon tree size diversity index, Simpson’s reciprocal index and evenness index, all applied to diameter classes) and the abundance of large living trees. For all silvicultural systems investigated, stand volume and harvested timber volume have grown since the beginning of the management plans (beginning of the twentieth century in some forests). The largest yields of timber corresponded to the best quality sites with more intensive silvicultural treatments (uniform and group shelterwood systems). The uniform shelterwood system showed lower figures for structural diversity, though not always significant. The best site qualities maintained notable structural diversity values, even under the most intensive management system. The application of the different management systems over decades has revealed a synergy between timber production and structural diversity, particularly in those systems maintaining more than one age class, although results are conditioned by forest harvesting history. The interaction between historical silvicultural treatment and site quality has been identified as an important source of information to understand forest dynamics and functioning of ES provision.

Impact of operational factors on fossil energy inputs in motor-manual tree felling and processing: results of two case studies

In many cases tree felling and processing operations are carried out motor-manually and knowledge about fossil fuel consumption and direct energy inputs when using such equipment is required for different purposes starting with operational costing and ending with environmental assessment of forest operations. In this study, fuel mixture, chain oil and direct fossil energy inputs were evaluated for two chainsaws which were used to fell and process trees in two silvicultural systems. The results of this study suggest that there is a strong dependence relation between selected tree size variables such as the diameter at breast height and tree volume on one hand and the fuel mixture, chain oil and direct fossil energy inputs when felling and processing broadleaved hardwood and resinous softwood trees on the other hand. For the broadleaved trees (mean tree volume of 1.50 m3 × tree-1, DBH of 45.5 cm and tree height of 21.84 m) the mean direct fossil energy input was of 3.86 MJ m-3 while for resinous trees (mean tree volume of 1.77 m3 tree-1, DBH of 39.28 cm and tree height of 32.49 m) it was of 3.93 MJ m-3. Other variables, including but not limited to the technology used, work experience and procedural pattern, may influence the mentioned figures and extensive studies are required to clarify their effects.

An overview of maritime pine private non-industrial forest in the centre of Portugal: A 19-year case study

Abstract Portuguese national policies for forests were developed considering related themes such as climate change, forest health, fire and the protective functions of forests. In Portugal, maritime pine forest is mainly private non-industrial and its area is in decline. Therefore, the aim of this study was two-fold: first, to assess maritime pine forest characteristics over a 19-year period; second, to analyse forest cover change over that period. In the end, the implementation of state policies was explored. A study area highly forested by continuous areas of naturally regenerated maritime pine in the centre of Portugal was used. To assess maritime pine forest characteristics, two sets of inventory data collected in previous studies (1991-1996 and 2007-2010) were used. To analyse forest cover change, the official land cover maps for 1990 and 2007 were used. This study findings highlighted that study area’s trends over the past years were the following: first, the decrease of maritime pine forest areas and its management decline (stands less stable, under-stocked, with large amounts of small-diameter poles and enlarged tree size variability); second, the increase of scrubland areas; third, the increase of eucalyptus afforestation with no regard for protection areas; and fourth, the absence of native oaks or introduction of other broadleaves as recommended by the state policies. Therefore, it is argued that there is a need for effective field monitoring actions with regard to the implementation of state policies. Additionally, selective incentives are key to mobilise private non-industrial forest to achieve the goals of state forest policies.

Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro)

Abstract. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.

Development of stand structural heterogeneity and growth dominance in thinned Eucalyptus stands in Brazil

Higher tree size heterogeneity in forests has been linked to lower productivity, especially in monocultures. Thinning the stand can directly manipulate tree size variability, thereby changing growth partitioning across tree dominance classes and consequently changing the development of stand structural heterogeneity compared to its course in unthinned stands. We used three thinning-intensity experiments to assess how stand heterogeneity and growth dominance develop through time and across different thinning intensities in Eucalyptus stands in one of the most productive regions of Brazil. The experiments were established along a three-site gradient in productivity. The plots were thinned from below at ages 58 and 146months. The thinning intensities included the removal of 20%, 35% and 50% of basal area and an additional treatment of 35% removal plus pruning concomitantly to thinning. Thinning reduced stand heterogeneity and growth dominance as well as their development through time and this reduction was greater the more intense the thinnings were. Stand structural heterogeneity and growth dominance both increased before and after the first thinning, but the rates of increase after the first thinning were lower than they were before the first thinning. After the second thinning, heterogeneity tended to remain constant, whereas growth dominance tended to decrease, reaching negative values. This was contrary to our expectations of observing the same trends in both variables. Our results show that thinning from below not only reduces heterogeneity and growth dominance by removing the smaller trees, but can also slow the development of stand heterogeneity and growth dominance in Eucalyptus stands and potentially other monocultures.

Individual tree size inequality enhances aboveground biomass in homegarden agroforestry systems in the dry zone of Sri Lanka

Individual tree size variation, which is generally quantified by variances in tree diameter at breast height (DBH) and height in isolation or conjunction, plays a central role in ecosystem functioning in both controlled and natural environments, including forests. However, none of the studies have been conducted in homegarden agroforestry systems. In this study, aboveground biomass, stand quality, cation exchange capacity (CEC), DBH variation, and species diversity were determined across 45 homegardens in the dry zone of Sri Lanka. We employed structural equation modeling (SEM) to test for the direct and indirect effects of stand quality and CEC, via tree size inequality and species diversity, on aboveground biomass. The SEM accounted for 26, 8, and 1% of the variation in aboveground biomass, species diversity and DBH variation, respectively. DBH variation had the strongest positive direct effect on aboveground biomass (β=0.49), followed by the non-significant direct effect of species diversity (β=0.17), stand quality (β=0.17) and CEC (β=−0.05). There were non-significant direct effects of CEC and stand quality on DBH variation and species diversity. Stand quality and CEC had also non-significant indirect effects, via DBH variation and species diversity, on aboveground biomass. Our study revealed that aboveground biomass substantially increased with individual tree size variation only, which supports the niche complementarity mechanism. However, aboveground biomass was not considerably increased with species diversity, stand quality and soil fertility, which might be attributable to the adaptation of certain productive species to the local site conditions. Stand structure shaped by few productive species or independent of species diversity is a main determinant for the variation in aboveground biomass in the studied homegardens. Maintaining stand structure through management practices could be an effective approach for enhancing aboveground biomass in these dry zone homegarden agroforestry systems.